1. Field of the Invention
This invention relates generally to apparatus for processing digital data and, more particularly, is directed to an apparatus for processing digital data obtained from a digital video tape recorder performing a reproducing or playback operation in a shuttle mode, that is, with a variable tape speed.
2. Description of the Prior Art
When a video signal is reproduced at a variable tape speed by a digital video tape recorder (VTR) which reproduces the data of one field by scanning a plurality of oblique tracks, a large time base displacement or error occurs between a reproducing reference signal and the actual reproduced video signal, and such time base error must be corrected.
Therefore, in the prior art, words or blocks of data and the respective addresses are extracted from the reproduced data and the words or blocks of extracted data are then written in a memory at addresses therein corresponding to the respective extracted addresses. Then, the written data is read out from the memory in accordance with read addresses generated in response to a reference signal. The data thus read out from the memory are subjected to a suitable signal processing, such as, error detection and correction in a corresponding circuit, and then transmitted. An example of a digital VTR capable of playback at a variable tape speed is disclosed in Japanese laid open Patent Application Nos. 56-144682 and 60-40573.
In the prior art digital data processing apparatus referred to above, any data once written at a particular address in the memory is held thereat until new data is written at such address in the memory. Accordingly, if data is written in the memory for an address of the field which is not reproduced for a relatively long period of time, for example, as when a reproducing operation is being effected with a tape speed that is twice the normal tape speed, such data is retained in the memory and eventually has no correlation with other portions of the picture or image represented by the data in the memory. In such case, when the data in the memory are read out therefrom in the order of the read addresses generated in response to the reference signal, the resulting picture or image becomes unclear or erroneous in that a residual image appears therein, and the motion of the displayed picture is unnatural or lacking in smoothness.